1. Field of the Invention
This invention relates to methods and devices for determining the internal structure and composition of objects using small angle deflection of penetrating radiation.
2. Description of Related Art
Known absorption radiography devices determine the internal structure of an object by recording the intensity distribution of radiation transmitted through the object. Variations in the intensity distribution result from differences in radiation absorption in different paths through the object. For such devices, scattering of radiation in the object creates background noise and reduces image contrast. To offset the effect of scattered radiation, U.S. Pat. No. 4,651,002 proposed recording the scattered radiation separately using a collimator and filter, and subsequently subtracting the recorded scattered radiation intensity from the overall intensity distribution obtained when the object is X-rayed. U.S. Pat. No. 4,651,002 measures an integral of scattered intensity which does not require a fine adjustment of the relative positions of collimation and filter lattices. Accordingly, the filter is implemented as a mobile element, and the scattering is measured for large angles.
To account for background scattered radiation, U.S. Pat. No. 4,549,307 proposed special lids that block incident radiation and form spots on an object being inspected. In the spots, only the background i.e., only the scattered radiation is recorded. The background level over the entire image is approximated from measurements in the spots and is subsequently subtracted from the overall absorption signal to obtain a higher-contrast image.
As mentioned, the above-described devices identify or determine the internal structure of an object from the intensity distribution of the radiation transmitted without deflection through the object. If the object contains substances only weakly differing in absorbing properties, an image obtained using these devices may lack the contrast required to distinguish the parts of the object containing such substances, and imaging the object's internal structure may require an approach other than absorption radiography.
British patent No. 2,299,251 (1996) proposed a device using Bragg reflection from the crystal structures to identify crystalline and poly-crystalline substances. A collimator of the device allows recording of the energy spectrum for each separate region of an object through which the radiation passes. The energy spectrum distribution of the polychromatic radiation reflected at a selected angle is characteristic of the crystal structure of the substance reflecting the radiation and hence allows identification of the substance using a database of energy spectrum distributions. This method was proposed for identifying explosives in luggage. However, the method is limited to detecting substance with a crystalline or polycrystalline structure.
U.S.S.R. patent document SU 1402871 (1987) and Russian patent document RU 2012872 (1994) describe devices for imaging an object's internal structure using the effects of X-ray refraction at boundaries between parts of the object with different electronic densities. Refraction deflects X-rays by up to three seconds. These devices use single crystals to collimate the incident radiation and filter the refracted radiation. A drawback of these devices is their dependence on single crystal reflection according to the Bragg law which causes small aperture ratios. For every wavelength, the radiation is reflected at a certain angle within a deflection interval equal to the angular interval of the Bragg reflection, about ten angular seconds. This means that only a fraction (about 10.sup.-5) of the source radiation energy is used for imaging the object.
Published PCT patent application No. WO96/17240 (1996) describes devices that achieve larger aperture ratios using aperture lattices instead of single crystals. In such devices, a collimation lattice before the object forms an incident flux as a series of narrow, weakly diverging beams. A filter lattice between the object and a detector acts as a scattering radiation filter. The two lattices are positioned with respect to each other so that the penetrating radiation flux in the absence of an analyzed object would not reach the detector. During imaging, the object is immobile with respect to the detector, and the spatial frequency and positions of the detecting rays determine the positions and sizes of the X-rayed parts of the object. The collimation lattice is preferably large enough to encompass the entire object and should have the opaque regions no more than 0.05-0.1 mm wide to ensure a suitable resolution for detecting inhomogeneities in the analyzed object. These two requirements of the collimation lattice increase the device cost and complicate adjustments of the device.
U.S. Pat. Nos. 4,751,772, 4,754,469, 4,956,856, 5,008,911, and 5,265,144 describe methods and devices for examining biological tissues and identifying explosives in luggage by recording the spectra of coherent radiation scattered at angles within 1.degree. to 12.degree. of an incident beam direction. A large part of the elastically scattered radiation is within those angles if the X-ray energy is small enough. As specified in those patents, analysis of the object uses a narrow collimated beam of monochromatic or polychromatic radiation. The intensity of the coherently scattered radiation is measured using a detecting system resolving both the energy and the scattering angle of the radiation. Several principles underlie these devices, one of which is that the energy spectra of elastically scattered radiation (unlike inelastically scattered Compton radiation) are identical to the spectrum of the primary beam. The intensity of the elastically scattered radiation has a characteristic angular variation with a pronounced maximum in the angular interval from 1.degree. to 19.degree.. The maximum defection angle depends on the X-rayed substance and the energy of the incident radiation. Since the intensity distribution of the coherently scattered radiation for small scattering angles depends on the molecular structure of the object substance, substances with the same absorbance (which conventional absorbance X-ray analysis cannot distinguish) can often be discriminated by the intensity distribution of the angular scattering of coherent radiation.
U.S. Pat. Nos. 4,751,722 and 4,754,469 describe devices using small-angle coherent scattering and computer tomography to form an image. The described devices have relatively low sensitivities since the coherent scattering cross-section is small in the specified angular range, therefore high radiation doses are required to X-ray an object. U.S. Pat. No. 5,265,144 describes a device using concentric detecting rings for recording the radiation scattered at particular angles. That device X-rays an object using a narrow beam with a small divergence, which is required for successfiul recording of the small-angle scattering, and has the problem of the small aperture ratio and, consequently, low sensitivity. The radiation flux in the described devices scatters off of different materials encountered during passage through the object, so that intensity distributions are superpositions of several curves resulting from different materials contained in the object. This complicates the substance identification from scattering curves. U.S. Pat. No. 4,752,722 proposes solving this problem using small-angle computer tomography. However, forming a tomographic image requires X-raying an object from a large number of different angles (0 to 360 degrees) which is expensive and not always feasible.
This invention aims to obtain information on substance distribution over the volume of an analyzed object using a relatively inexpensive device having a high aperture ratio. Further, the invention aims to create a device that is easier to produce and operate while having enhanced image quality when imaging or forming a projection of an object's internal structure.